1. Field of the Invention
This invention relates generally to memory device packaging and, more particularly, to high density digital memory cards having application for USB drive storage, flash and ROM memory cards, and other memory cards of similar format.
2. Background Art
Most conventional memory card assemblies (excluding the outside casing) include a rigid laminate substrate, such as an FR-4 printed wiring board (PWB), a controller, the required passive components, and at least one digital (e.g., flash) memory storage device in a TSOP (thin small outline package) type 1 package. The TSOP 1 packages are desirable because of their thin profile, typically only 1.2 mm high, and small overall footprint that can meet the size limitations of a memory card. While a memory card may vary in size and shape, most have limited thickness in order to be able to fit into the slot of a reader that is adapted to receive specific types of memory cards. For example, a standard compact flash (CF) card has an overall thickness of 3.3 mm, a secure digital (SD) card has a thickness of 2.1 mm, and a multimedia card (MMC) is only 1.4 mm thick. Even a relatively bulky USB drive storage card has an overall thickness that is typically less than 10 mm.
Because of the thin profile restriction established by the reader in which the memory card must fit, the overall thickness of the substrate/package assembly is confined to that of a thin form factor. Typically, a monolithic (i.e., single) package assembly is arranged on one side of the substrate. To double the density of the memory, a TSOP package may also be assembled on the opposite side of the substrate.
To overcome the space constraint, while increasing the memory density, the typical solution has heretofor been to use a stacked-die TSOP package, wherein two or more flash ICs are stacked one above the other inside a single package. A stacked-die TSOP package may retain its overall outside dimension, including thickness, by using two thinned ICs inside the package. Therefore, during assembly, the overall profile of the stacked-die package will remain the same as the thickness of a single die package.
However, such a stacked-die package is more difficult and expensive to manufacture. First, the IC chip must be thinned to less than one half the normal IC thickness in order to fit inside the package. In addition, the package assembly procedure is more complicated than that used for a standard, single-die package assembly. Finally, the assembled package must undergo functional testing. If one of the two IC devices fails, the standard-die package may have to be downgraded to the equivalent of a single-die package. In other cases, the stacked-die package having a single defective IC device may become entirely unusable. Consequently, if the test yield is low, it can prove to be costly in situations where stacked-die packages are used to achieve high density for memory cards.
Accordingly, what is needed is a high density memory card assembly (e.g., for use as a USB drive or flash storage memory card) that preferably uses a standard single die package that is a readily available industry standard so that the testing and assembly of the package follow established procedures that are widely known and of relatively low cost.